SP_disconnect should be called when the application is finished
with a connection to the Spread daemon. The application may have
other connections still open to the daemon and may open a new
connection after disconnecting.
If any SP_ function returns a
NET_ERRROR_ON_SESSION error on any call, then the mailbox cannot be used anymore
until it is disconnected with a call to this function and then
reconnected by calling SP_connect(). Any other SP_ calls
will only immediately return the same error value.
mbox should be for the connection you wish to disconnect from.